x86/AMD: Load context when attempting to read VPMU MSRs

x86/AMD: Load context when attempting to read VPMU MSRs

Load context (and mark it as LOADED) on any MSR access. This will allow
us to always read the most up-to-date value of an MSR: guest may write
into an MSR without enabling it (thus not marking the context as RUNNING)
and then be migrated. Without first loading the context reading this MSR
from HW will not match the pervious write since registers will not be
loaded into HW in amd_vpmu_load().

In addition, we should be saving the context when it is LOADED, not
RUNNING --- otherwise we need to save it any time it becomes non-RUNNING,
which may be a frequent occurrence.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Dietmar Hahn <dietmar.hahn@ts.fujitsu.com>